In thermal transfer printing, a final support surface such as a cut sheet of paper or the like is held closely adjacent to an ink donor surface, such as an ink carrying film, to allow the transfer of ink from the donor surface to the final support surface for printing an image. The inks used in thermal transfer printing are normally in a solid condition, and are subject to melting on the application of an appropriate amount of heat energy. In liquid state, the ink flows onto the final support surface. Upon removal of the source of heat energy, the spot of ink resolidifies and bonds to the final support surface, providing a visible image thereon. The process produces acceptable print quality, at reasonable cost and speed. It is a desirable feature of thermal transfer printing devices that they are very quiet relative to impact printers.
A thermal transfer printing device includes a printhead comprising a thermal element composed of an array of selectably controllable resistive heat producing elements, each element constituting a pixel in a line to be produced on the final support surface. The array is supported in closely spaced relationship to an ink donor surface to supply heat energy to melt the solid ink deposited thereon. The array is generally arranged to print across the width of an entire sheet, so that a 300 spot per inch printer will have approximately 2650 elements to print across an 8.5 inch wide sheet. A voltage is controllably applied to each individually addressable resistive element, in accordance with a stored electronic image, to energize the element to melt the ink on the donor surface in an area local to the element to form a dot on the support surface. Relative movements of the printhead, the ink donor surface and the sheet allow the movement of the imaging process along the sheet to produce a series of lines to form the complete image.
Resistive heating element arrays are commonly formed in a layering process in which a resistor is deposited on a substrate using thin or thick film techniques. The substrate is typically 1mm thick alumina (Al.sub.2 O.sub.3) which provides an electrically insulating, thermally conductive substrate. Between the alumina and the resistors is a glaze layer, comprising a thermally isolating glass or ceramic material about 50 .mu.thick. The alumina substrate is adhesively bonded to an electrically and thermally conductive metallic base, such as aluminum, about 5 mm thick, for strength, and also to provide a heat sink for the printhead and an electrical ground for the resistor array. The glaze layer provides thermal isolation so that the resistor can reach a peak temperature of 300.degree.-400.degree. C. within a millisecond of the application of a power input of about 0.5 Watts, but also allows enough thermal conductivity for the resistor to cool below the melting point of the ink within a few milliseconds after power is removed from the resistor. The alumina substrate serves to disperse the heat from the glaze layer to a sink very quickly, and is a useful substrate material in fabricating the resistive heating element arrays. These structures are not optimal, however, because of the several layers required to fabricate the printhead (four counting the adhesive layer required to bond the alumina to the aluminum), inefficiency in heat transfer characeteristics and limitations to printing resolution.
Conventional thermal printheads, whether fabricated by thick film or thin film techniques, rely upon the glaze layer to thermally isolate the resistors from the alumina substrate. The glaze layer, typically Corning 0080 glass or its equivalent, and about 50 microns in thickness, provides enough thermal isolation that, when driven, the resistors reach an operating temperature of around 300.degree. C. and cool, upon the removal of heating power, to a temperature less than the melting point of the ink at approximately 60.degree. C. in a few milliseconds. The thickness of the glaze layer is dictated by these requirements, and by the fact that the thermal conductivity of Corning 0080 glass is relatively high, in the range of 2 .times.10.sup.-3 cal/sec-cm-.degree. C. The glaze layer material is isotropic, so that a spherically symmetric thermal bubble (isotherm) propagates from a point source of heat (the resistor). The isotherm propagates in accordance with ##EQU1## where k is thermal conductivity;
t is time; and PA1 .rho.C is the volumetric specific heat.
The time scale of operation of the heating element is set by the requirement that the isotherm, with a temperature necessary to melt the ink, and a size necessary to produce a fully formed pixel, propagate through the ink donor film. The period taken for this to occur is about 2.5 milliseconds. During this same period, however, the same isothermal surface diffuses through the glaze layer, and heat is lost to the alumina sink. The thickness of the glaze layer is selected in part to minimize heat loss to the sink from occurring. However, the isotropic nature of the material also allows lateral diffussion of heat. Lateral diffusion of heat in the glaze layer limits the resolution attainable with thermal transfer printing.
In U.S. Pat. No, 4,296,309 to Shinmi et al. the thermal printhead site generally comprises an aluminum base, an alumina substrate over the base, a glass layer over the alumina substrate and supporting a resistive heater element, an electrode driving the resistive heater element and a protective overcoating. Japanese Patent No. 59-174370 shows a thermal printhead site including an iron or aluminum heat dissipating layer, an insulating layer, a heating element, a conductive layer connected to the electrode and a protective overcoating. United States Patent No. 4,561,789 to Saito shows a similar thermal printhead site. United States Patent No, 4,030,408 to Miwa shows resistive elements seated in recesses in a ceramic base and covered with a protective overcoating.
Polyimides are organic heat resistant materials, having a lower thermal conductivity than conventional glaze layer materials. Additionally, polyimides may be applied in thin layers and are photo definable so that they are useful in thin film deposition techniques of manufacturing. U.S. Pat. No. 4,561,789 to Saito shows the use of polyimides in a porous printhead for thermal ink transfer printing suggesting the use of a polyimide insulation film between an electrode and an aluminum substrate in that configuration, but does not teach how the polyimide layer can be used as an improvement over a glaze layer for the control of the melting isotherm expansion in printheads.
To achieve the higher resolutions desirable for high quality printing, a large number of resistor elements per unit of length is desirable, since the greater the number of addressable locations in the image, the finer the image may be made, with fewer jagged edges and print artifacts. A known method of resistor spacing, referred to hereinafter as interdigitation, places the resistors in two closely spaced parallel arrays, each resistor element in an array being placed in a position opposite to a space between adjacent resistors in the opposing array. The close spacing of the array and the width of the printing nip, in combination with the dot size produced by each element, allows the appearance of a straight line produced by the resistors when the appropriate time delay between the two arrays is used to drive the resistors. Overdriving of the resistive heating elements is not required to heat the area between the resistors to melt ink on the donor surface in the space between the resistors, thereby increasing life and decreasing power supply requirements. Interdigitated arrays of this type are shown in Japanese Patent No. 56-118879, Japanese Patent No. 59-93367 and U.S. Pat. No, 4,030,408 to Miwa. The structure of these arrangements, however, requires multiple levels of circuitry for connection of the resistors to the common bus and ground plane. It would be desirable if an arrangement could directly connect the resistors to a common bus, connected directly to a massive current carrying metallic substrate.